The pump cavity of a solid state laser, in order for the laser to emit electromagnetic radiation, must be excited by an outside source of energy. This source of energy emits radiation itself which is converted by a laser rod within the pump cavity and the laser's other optical components into a laser beam. This source of energy may be a laser lamp or other optical energy sources. Solid state laser rods, as generally true of all laser media, convert much of the absorbed pump power into waste heat. This heat and the thermal gradient induced stresses associated with its removal lead to optical distortion in the form of lensing and birefringence, and ultimately to thermally induced fracture and failure. Thus, in order to counteract these heating problems, a variety of thermal management techniques are employed in the prior art to dissipate the generated heat in an appropriate manner. For example, if the heating is more than about 3 watts (about 35 watts of electrical power to a lamp used to provide pump energy), the laser rod can be cooled by flowing a fluid, such as a liquid or pressurized gas, over the rod. The fluid is recycled through a heat exchanger. Such a dynamic cooling system has various drawbacks. In the case of pressurized gas, a major problem is the high cost of sealing the circulating coolant gas while still permitting transmission of the light from a pump lamp. For liquid systems, the concern is the ultimate deterioration of the fluid upon exposure to the lamp, or pump light.
Because of these problems, another technique for cooling the rod is related to passively cooling the rod by thermal conduction. This technique is exemplified in U.S. Pat. No. 4,210,389 to Burkhart et al, in which the rod is cooled by depositing a reflective metallic layer on one side of the rod, which is soldered to a heat sink. This technique has been found to grossly distort the optical quality of the rod for at least two major reasons. First, because the rod is rigidly held in a thermally conductive support and the rod and the support have different coefficients of thermal expansion, stresses are developed during pumping and cooling; therefore, the rod expands and contracts at a rate different from its support. This unequal expansion and contraction induces stress birefringence. Second, the rod is supported on the support along its length on only a portion of its periphery, which creates a thermal gradient within the rod and a consequent nonuniform cooling. The result is an introduction of optical aberrations in the rod.
U.S. Pat. No. 4,181,900 to Tajnai et al also describes a conductively cooled pump cavity, in which the laser rod is strapped to a heat sink. The Tajnai et al system does not provide a uniform technique for cooling the laser rod since there is no thermal contact around the entire periphery of the rod. The result, like U.S. Pat. No. 4,210,389, is uneven rod cooling with induced thermal stresses. In addition, the straps themselves mechanically stress the rod.
An alternative approach for conductively cooling a laser rod is disclosed by Kahan in U.S. Pat. Nos. 4,637,028 and 4,969,155 and by Gregor in U.S. Pat. No. 5,317,585. These patents disclose a laser rod which is conductively cooled through a transparent outer sleeve, with an intermediate layer of gel used to form an elastomer interface between the rod and the gel. The transparent outer sleeve is preferable made of sapphire, but it may also be made of other optically transparent materials such as glass, single crystal beryllium oxide, and garnet.
Assembly of this apparatus is quite complex. During the manufacture of the apparatus, the laser rod must be held in place with shims within the outer sleeve, until a gel-forming liquid is injected into the sleeve. Once the gel is formed, the shims are removed. The elastomer, since it forms a thermal boundary, must be kept quite thin in order to maintain adequate heat conduction. The thinness of the layer additionally complicates the manufacture of the apparatus. Since the elastomer completely surrounds the laser rod, it must be transparent to pump light to allow for excitation of the laser rod. There are only a limited number of materials suitable for transmission of pump light. Kahan discloses the use of a silicone gel, but this type of gel exhibits out-gassing characteristics, therefore requiring a sealant to be applied to the end boundary points to avoid affecting the laser optical components. Finally, the outer sleeves are generally available only in limited lengths, therefore the design often requires that two sleeve segments be joined together, which additionally complicates the manufacturing process.
Additional concerns with solid state lasers are Amplified Spontaneous Emission (ASE) and parasitic oscillations. ASE occurs when spontaneous laser radiation is amplified during a single pass through the laser media, while parasitic oscillations are ASE which is reflected back into the laser rod. In either case, the energy available for extraction in the preferred direction is reduced. The elastomer disclosed by Kahan and Gregor is generally not a good index match to the rod material, therefore the elastomer provides little suppression of ASE and parasitic oscillations. Suppression of ASE and parasitic oscillations requires that the surface of the rod be fine ground, but this surface grind partially back scatters the pump energy, thus reducing pumping efficiency. Gregor discloses the use of a coating to suppress ASE, but the elastomer layer is located between the ASE-suppressing coating and the laser rod, thus limiting the effectiveness of the coating. This outer coating must be designed to be highly reflective at the pump wavelength and transmissive at the laser wavelength, which requires an expensive multiple layer dielectric coating,
Therefore, there exists a need in the art for a method and apparatus for conductively cooling a solid state laser rod which can provide for suppression of amplified spontaneous emission and parasitic oscillations.